It is common practice to employ synchronization or "sync" characters in data storage devices that, when recognized in a data stream by an associated device controller, establish the boundaries of various fields within the data stream. These sync characters are commonly preceded by a preamble used to generate a sampling clock in the controller. A good example of this practice is shown in U.S. Pat. No. 5,036,408, issued Jul. 30, 1991 to Leis, et al., assigned to Digital Equipment Corporation, and entitled "High Efficiency Disk Format and Synchronization System" (hereinafter referred to as "Leis"). Therein is described a synchronization system that employs two sync characters: one in a header field used in reading the header, and another in a data field used in reading the data. These sync characters are traditionally copies of the same sync pattern. A single correlator designed to recognize the sync pattern is used to establish both header and data synchronization under the direction of a higher-level controller.
It is very important to establish synchronization in a reliable fashion. A device controller relies on sync detection to establish precisely where meaningful data resides in the data stream. If synchronization is done incorrectly, the retrieved data usually bears no resemblance to the correct data. This could have catastrophic results, for example, if it results in unpredictable behavior of the storage device. In such a case, valuable user data might be mistakenly destroyed. In other cases, the mistake is discovered before such catastrophic behavior, but the performance of the storage device has degraded at least temporarily. Such results are clearly best avoided.
Because of the need for reliable sync detection, then, the patterns used for sync characters are very special. They must be as different as possible from cell patterns preceding the sync character in the data stream. In this way the reliability of synchronization is enhanced.
In any system, such as that of Leis, that employs two copies of the same sync pattern in different fields, there is a chance that one field will be mistaken for the other during operation, with very deleterious results. For example, in the system of Leis a false detection of a DC erase area and a preamble preceding the header can occur. When a subsequent data sync character is detected, the controller can mistakenly interpret it as a header sync character, believe that it is operating on a header, and interpret user data as address information. Such a mistake could cause incorrect operation or additional delay in establishing correct synchronization.